Image formation process in electrophotography generally comprises a step of forming an electrostatic latent image by uniformly charging and, thereafter, imagewise exposing a photoconductive photoreceptor, a step of developing thus formed electrostatic latent image with a toner, which is formed of a fine resin powder containing therein a colorlant, etc., a step of transferring thus formed toner image onto a transferee such as transfer paper, and a step of fixing the toner image by means of heating, pressurizing, etc., to obtain a visualized image.
The image forming process making use of the electrostatic latent image should preferably be performed as quick as possible, and in this view, the heat roll fixing process, which has better heat efficiency and other advantages in comparison with other fixing processes of the art, has been widely applied in this field.
Recently, however, further speed-up of the total image formation process is strongly demanded, and for the attainment of this purpose, it becomes essential for the fixing step to be speeded up. For the fixing of toner to be carried out at higher speed in the heat roll fixing method, the toner for the development is required to have a fixability at lower temperature, and for this purpose, the binder resin to be used should necessarily have a lower softening point. However, in general, when the softening point of the binder resin of a toner is lowered, a part of the toner image tends to be transferred to the surface of the heat roll and thus transferred toner is re-transferred to the surface of the subsequent transfer sheet, which is so-called an off-set phenomenon, to cause deterioration in the toner image. In addition, in the heat roll fixing method there is a tendency that a transfer sheet easily winds round the surface of the heat roll, which is so-called a winding penomenon. This winding phenomenon is likely to take place especially when the temperature of the heat roll is too low.
For the purpose of preventing the off-set or the winding phenomenon from taking place, various measures have been proposed and some of them have been put into practice. As one of such measures, it has been proposed that fixing be carried out while providing to and coating the surface of heat roll with a mold releasing oil such as silicone oil. The another proposed measure is to confer the toner itself with mold releasing property and thus to give it the off-set prevention property. The latter measure has an advantage over the former that no oil providing system in the copying machine is necessary and, therefore, the structure of a fixing unit could be made simpler and that no maintenance, such as repleniching of silicone oil, is needed, etc.
As for the means for conferring the toner itself with good off-set prevention property there has been proposed a method of (a) using as a binder resin for the toner a polymer having high molecular weight, or (b) incorporating a mold releasing compound into the toner. However, by means of method (a), although the off-set prevention property of the toner may be improved, due to the rise of the softening point of the resin, it becomes difficult to carry out fixing operation at low temperature.
Recently as a binder resin having low softening point, polyester resin has been drawn attention. By the use of a polyester resin, as low molecular weight resin being readily obtainable, it is possible to obtain a toner having relatively low softening point. The toner using the polyester resin as a binder has, in comparison with the toner in which a vinyl-type polymer is used, such an advantage that it has better wetting property against a transfer sheet such as paper and, therefore, fixing at lower temperature becomes possible comparing with the case where a vinyl-type polymer having approximately the same softening point is used.
On the other hand, as for method (b), wherein toner is incorporated with a mold releasing agent, it has been proposed, as disclosed in Japanese Patent Publication No. 3304/1977 or 3305/1977, to use as a mold releasing agent a polyolefin wax such as a polypropylene or polyethylene. It is generally effective to use such mold releasing agent, however, in order to make a toner having sufficient off-set prevention and winding prevention properties as well as wide practical fixing temperature range, the proportion of the mold releasing agent to be incorporated must be increased to a considerable extent. However, with the increace of the proportion of such mold releasing agent in the toner, fluidity of the toner powder comes to be lowered, and as the result, satisfactory development becomes difficult despite of the improvement in the fixing operation and it becomes difficult to obtain an toner image of good image quality.
Further, with the use of a toner containing a polyolefin wax, there has been observed a disadvantage that, when such toner is used together with carrier particles in a so-called two-component developer, the carrier particles are liable to wear out easily and the effective life time of such carrier is shortened. This is considered due to so-called filming phenomenon by adhesion of the wax component in the toner to the surface of the carrier particles. Reason for the occurrence of the toner filming is considered to reside in the the fact that it is necessary to increase the content of such wax in the toner in order to attain sufficient off-set prevention effect, and in addition, since the dispersion capability of the resin of such wax is relatively low and the domain of the wax becomes too large, the wax is considered to be present in the toner resin in a separate phase.